1. Field of the Invention
The present invention relates to a method for drying plant-derived biomass in an extremely economical way, and also to a method for producing biomass fuel by using said drying method.
2. Description of the Prior Art
The term “biomass” has been defined originally as “the amount of living matter” in the field of biology. At present, however, it is used in a broader sense to imply foods, materials, and fuels composed of substances originating from organisms.
Although biomass includes those such as rice bran and charcoal which are in routine use, it also includes increasing industrial wastes whose disposal is under study. Wastes from agriculture include coconut husks and rice hulls; wastes from forestry include wood chip dust and pruned branches; wastes from animal husbandry include fecal matter; and wastes from fishery include internal organs and bones remaining after processing. Food wastes and sewage sludge are also regarded as biomass, and the way of their disposal is not yet established.
It is essential that the biomass mentioned above should be disposed of at a low cost. A desirable way to dispose of biomass is to reuse biomass as fuel. Unfortunately, biomass contains a large amount water and hence needs to be freed of water (or dried) before its reuse. At present, however, the drying step is so expensive as to make reuse impracticable.
No. 1. Japanese Patent Laid-open No. 139574/1975 (page 1)
No. 2. Japanese Patent Laid-open No. 165490/1988 (claim 1 and others)
No. 3. Japanese Patent Publication No. 41268/1995 (claim 1 and others)
No. 4. U.S. Pat. No. 4,959,154 (page 2, right column, lines 30-36)
No. 5. U.S. Pat. No. 5,244,472 (claim 1 and others)
No. 6. Japanese Patent Publication No. 447 18/1983 (page 1)
No. 7. Japanese Patent Laid-open No. 26289 1/1985 (claim 1 and others)
No. 8. Japanese Patent Publication No. 15560/1989 (claim 1 and others)
No. 9. Japanese Patent Laid-open No. 27858 1/1995 (claim 1 and others)
Some methods for drying sewage sludge as biomass are disclosed in the patent literature Nos. 1 to 3. Despite their respective features, they involve in common a step of heating sewage sludge in oil under reduced pressure or atmospheric pressure, thereby reducing thermal energy required for drying.
As for disposal of biomass other than sewage sludge, a method for drying wood chips used to clean up oil spill is disclosed in the patent literature No. 4. This method involves a step of keeping wood chips in oil at 121-177° C. for 3-12 minutes. A method of heating and drying wood chips in oil at 163-191° C. is also disclosed in the patent literature No. 5.
These drying methods, however, are not satisfactory when considered from the standpoint of reusing the dried biomass. In other words, heating biomass in oil under reduced pressure or atmospheric pressure surely achieves drying, but the resulting dried biomass as such does not serve as fuel, because plant-derived biomass is difficult to crush into small pieces due to its high content of fibrous components (such as cellulose) and is unsuitable for transportation due to its bulkiness. On the other hand, plant-derived biomass may possibly be mixed with coal or any other fuel; however, it will greatly impair the strength of fuel because it is not easily pulverized. Even if dried biomass in pulverized form is to be added to fuel, its amount is limited because simply dried biomass produces no binding effect.
Another conceivable way to eliminate the disadvantage of plant-derived biomass involving difficulties in pulverization due to fibrous materials is by heating in oil under conditions severe enough to bring about drying as well as decomposition. Plant-derived biomass such as food wastes and wood chips, which are composed mainly of cellulose, hemicellulose, and lignin, is decomposed into low-molecular substances (such as monosaccharide and monolignol) upon heating in oil at high temperatures under high pressures. Such decomposition products are not suitable as fuel constituents because of poor combustibility and high water solubility (which makes separation from water extremely difficult).
Meanwhile, fuels composed of wood powder and low-grade coal mixed together are disclosed in the patent literature Nos. 6 to 8. These fuels use wood powder as a binder, which leads to the advantage of having adequate strength and emitting less soot and smoke upon combustion.
The technologies disclosed in the patent literature Nos. 6 to 8, however, have completely overlooked the problem with drying biomass (wood powder). The technology disclosed in the patent literature No. 6 even proposes treatment with steam to impart binder activity to wood powder. Therefore, the fuel proposed in the patent literature Nos. 6 to 8 would be poor in combustion heat due to moisture contained therein.
The patent literature No. 9 discloses a technology of making solid fuel from residues of animals and plants by drying. This technology, however, completely overlooks the necessity of reducing the volume of plant-derived biomass.
As mentioned above, there have been several known methods for drying biomass. However, they are not necessarily suitable for application to plant-derived biomass from the standpoint of reusing biomass after drying. Moreover, they involve problems with recovery of oil used for drying and they are unable to process a large amount of bulky biomass. Fuel containing plant-derived biomass is known, but it is uneconomical, impracticable, and unsatisfactory because there has been no effective method for drying plant-derived biomass.